Quote Originally Posted by jiriku View Post
Immediate Action

Much like a swift action, an immediate action consumes a very small amount of time, but represents a larger expenditure of effort and energy than a free action. However, unlike a swift action, an immediate action can be performed at any time—even if it’s not your turn. Using an immediate action on your turn is the same as using a swift action, and counts as your swift action for that turn. You cannot use another immediate action or a swift action until after your next turn if you have used an immediate action when it is not currently your turn (effectively, using an immediate action before your turn is equivalent to using your swift action for the coming turn). You also cannot use an immediate action if you are currently flat-footed.

I often see people suggest that casting nerveskitter prior to your first turn in combat helps to ensure you go first. But how can you cast nerveskitter (an immediate action) before your first turn? You're still flat-footed, and therefore unable to take immediate actions.
Spell compendium errata.